Many types of fuel filters are known in the art. A popular type of fuel filter construction for self-propelled vehicles is one that has a housing which encloses a replaceable filter element. Fuel used to power the self-propelled vehicle, such as diesel or gasoline fuel, is cleansed of impurities as it passes through filter media in the filter element. The filter media captures many of the impurities that are removed from the fuel. Other impurities collect on the surface of the media and fall downward into a bottom area of the filter housing from where they may be periodically removed through a drain valve. Such a fuel filter is shown in applications U.S. Ser. No. 07/888,688, filed May 22, 1992 for "Fuel Filter Assembly with Reversible Element"; and U.S. Ser. No. 08/131/824 filed Jul. 27, 1993 for "Fuel Filter Element", which are assigned to the assignee of the present invention.
Through use, the filter media can become dirty and clogged. When the filter media becomes clogged, resistance to flow increases and the filter element must be replaced. Indicators such as pressure and water sensors have been used which provide the operator with an indication that a filter element needs to be replaced. However, since the self-propelled vehicle is not always in a position or location to be stopped and the filter element replaced, devices have been developed for allowing the fuel filter to continue operating in at least a reduced capacity until an appropriate time and location are found to replace the element.
One such filter which prolongs the use of the vehicle has a by-pass valve in the filter element which opens at a predetermined pressure to allow fuel to flow around a clogged filter media. This type of filter is shown in U.S. Pat. Nos. 4,322,290 and 4,246,109. The by-pass valve in the filter element opens a fuel flow path which entirely circumvents the clogged filter media. While this type of fuel filter enables a fuel system to remain in operation, the fuel which passes through the by-pass valve is entirely unfiltered and can adversely effect the fuel system.
An improvement in this type of fuel filter is shown in U.S. Pat. Nos. 2,998,138; 3,331,509 and 4,038,189. In these patents, a second filter media, separate from the first filter media, is provided in the flow path from the by-pass valve. The back-up filter media is not normally in the fuel flow path and hence is not used when the primary filter media is functional. The second filter media captures at least a minimal amount of impurities in the fuel until the filter element can be replaced.
Another improvement in this type of filter is shown in U.S. Pat. Nos. 3,000,505; 3,229,817; and 4,437,986. In these patents, fuel passing through the primary filter media also passes through a secondary filter media during normal operation. For example, in U.S. Pat. No. 3,000,505, the fluid normally passes through an outer wire cloth filter with relatively small pores and then through an inner metal edge filter with relatively larger pores. Fluid is primarily filtered by the outer filter until the outer filter becomes clogged. At this point, a slidable sleeve valve activates to provide a fluid flow around the clogged outer filter directly to the inner filter. Similarly, in U.S. Pat. No. 3,229,817, fluid flows through a central passage radially outward through a coalescing medium, and then upward through a wire mesh separating screen. The coalescing medium causes water in the fuel to bead and fall downward into a lower chamber. If the coalescing medium becomes clogged, a spring-biased ball-valve open a fuel flow path around the coalescing medium so that the fuel flows directly to the separating screen.
Finally, in U.S. Pat. No. 4,437,986, fuel flows radially inward through an outer coalescing medium which causes most of the water in the fuel to coalesce and fall downward into a sump. Fuel then flows through an inner separating membrane which is repellant to water and completes the separation of the water from the fuel. Wax crystals plugging the inner separating membrane increase the pressure differential across the membrane and cause a relief valve to open, which opens a flow path around the inner membrane.
While the above-described filters provide certain advantages for maintaining operation of the fuel system when a filter element becomes clogged or otherwise unusable, the present inventor has determined that further improvements are necessary for the fuel filter assembly. That is, the inventor has determined that the primary media in certain filter elements may degrade under certain conditions. For example, when the primary media is a water-coalescing media (i.e., one that causes water droplets in the fuel to bead on the outer surface of the media and fall downward into a catch basin or sump), the coalescing media can be susceptible to certain contaminants in the fuel, such as tar or asphaltenes. These contaminants are surfactants which can cause the primary media to absorb or become wetted with the water in the fuel, rather than coalescing the water. The water can eventually permeate the primary media and enter the fuel system. Under such conditions, the water-separating capabilities of the filter element can be quickly reduced, which can affect certain components in the fuel system (e.g., fuel injection equipment), without adequate warning to the operator.
The inventor therefore believes that there is a demand in the industry for a further improved type of fluid filter assembly which continues to separate and filter impurities in the fuel when exposed to contaminants and which adequately warns the operator of water passing through the filter element into the fuel system.